For any tall construction exposed to the elements, lightning protection is an important aspect. A tall construction must incorporate some way of conducting the electrical current to ground in the event of a lightning strike. In its simplest form, a lightning protection system (LPS) can comprise a lightning rod which is generally higher than the building to which it is attached, and which is connected to a metal strip or other electrical conductor leading directly down to ground. More advanced lightning protection systems comprise several conductors at extremities of the building or construction, and multiple conductive paths for guiding the electric current to ground.
Generally, the taller a construction, the more likely it is to be struck by lightning. Wind turbine towers are relatively tall constructions, often over 20 m in height, and are usually located in exposed areas such as on hilltops or offshore. Most wind turbine towers in use at the present time are constructed of steel, and the steel tower itself acts as a conductor.
However, steel towers can be costly to manufacture, since steel is becoming an increasingly expensive material. Furthermore, a steel tower must be assembled by connecting together several tower sections. Since tall towers are desired for several reasons which will be known to the skilled person, each section can be 10 m in length or more. The tower sections must be transported horizontally to the final destination where they are assembled using a crane. The transport of such long sections can be very difficult, since the roads along which they are transported cannot have any sharp turnings or low bridges. Therefore, before actually assembling a wind turbine towers in a certain location, it may be necessary to first build an access road. Obviously, this adds considerably to the overall costs.
For these reasons, it is becoming more attractive to construct wind turbine towers of an alternative material such as concrete, which is economical and easy to mix on-site. Concrete can be used in the construction of very tall buildings and towers such as television towers, and is given structural strength by the use of steel reinforcing elements such as reinforcing bars (also referred to as ‘rebar’) or post-tensioning cables (also referred to as ‘tendons’). However, unlike a steel wind turbine tower, a concrete tower requires an additional lightning protection system, since it is absolutely mandatory to avoid damage to the concrete such as that which can arise as a result of a lightning strike, in which secondary side-flashes can cause cracks in the concrete. Such structural damage can cause the concrete to crumble in places, thus severely weakening the structure. Furthermore, lightning current from a lightning conductor of the blades and upper wind turbine structure (hub, nacelle, etc.) must be conducted to ground in order to prevent lightning damage to electrical equipment.